Address multiplexed dynamic RAM having a test mode capability

ABSTRACT

An address multiplexed dynamic random access memory (RAM) which has both a normal operation mode and a test mode capability is provided. The test mode is initiated in response to particular signal level combinations of both the row address strobe (RAS) and column address strobe (CAS) signals and the write enable (WE) signal. Since the signal level combinations employed in connection with implementing the test mode are unused in the normal operating mode of the dynamic RAM, additional external terminals are unneeded. This dynamic RAM employs multiplexing circuitry on both the input side as well as on the output side of the dynamic RAM, which multiplexing circuitry is controlled during normal operation by select signals from a decoder and during the test mode by a test signal which allows accessing of data at all of the common complementary data lines by the testing circuitry so as to determine whether there is consistency or inconsistency of the data being read out for testing.

This is a continuation of application Ser. No. 07/648,885, filed Jan. 31, 1991, now U.S. Pat. No. 5,117,393, which is a continuation of Ser. No. 07/319,693, filed Mar. 7, 1989, now U.S. Pat. No. 4,992,985, which is a divisional of Ser. No. 07/041,070, filed Apr. 27, 1987, now U.S. Pat. No. 4,811,299.

BACKGROUND OF THE INVENTION

The present invention relates to a dynamic RAM (Random Access Memory) device and to the technology useful for fabricating a semiconductor memory with a storage capacity as large as 4 mega-bits.

Recent advances in semiconductor technology have enabled the development of a dynamic RAM as capacious as 1 megabits. RAM chips with such an increased storage capacity require an extended time period for testing. To cope with the testing of such increased storage capacity, there is known a dynamic RAM chip which is provided therein with a test circuit, and testing is conducted such that the same signal, which is in multiples of 4 bits, is written in different storage locations in the memory array and, if any one bit out of the signal retrieved from the memory array is inconsistent, the output terminal is brought to a high-impedance state. In the case where all bits of the read-out signal are high or low, the output terminal is enabled to output a high-level or low-level signal. (For details refer to the publication entitled "Mitsubishi Giho", Vol. 59, No. 9, pp. 60-63, published in 1985 by Mitsubishi Electric Corp.)

In the above test scheme, an unused pin on the 18-pin package is used to bring the RAM chip from the normal operating mode into the test mode in which the test circuit is activated. However, if a capacious dynamic RAM chip, e.g., a 4M bit RAM, is intended to be fabricated in conjunction with an 18-pin package, it uses up all spare pins to receive the address signal, and therefore the above-mentioned testing cannot be implemented.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method wherein a dynamic RAM device which can be tested in a shorter period of time without having an increased number of external terminals.

The present invention typically resides in a method for effecting a test mode capability in a dynamic RAM and in a dynamic RAM device having the capability of being placed in a test mode phase or condition upon detection of a low binary level of both the column address strobe signal and write enable signal when the row address strobe signal is at a transitional logic state or level corresponding to a falling or trailing edge. Accordingly, the test mode can be selected by the combination of the existing external control signals used in the normal operation, whereby time expended for chip verification can be reduced without requiring an additional external terminal.

These and other objects and novel features of the present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a timing chart used to explain an embodiment of this invention;

FIG. 2 is a block diagram of the dynamic RAM device embodying the present invention; and

FIG. 3 is a timing chart used to explain another embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows in block diagram an embodiment of the inventive dynamic RAM device. All circuit elements and blocks shown in the figure are formed on piece of semiconductor substrate, such as of p-type monocrystalline silicon for example, by a known complementary metal-oxide-semiconductor field-effect transistor (CMOSFET or complementary MOSFET) integrated circuit fabricating technique.

Each one bit memory cell (MC) of the RAM device consists of an information storage capacitor (Cs) and an access or transfer n-channel MOS FET (Qm) connected in series with the capacitor. Logical data "1" or "0" is memorized in the form of charges stored in the capacitor Cs. The capacitorCs has its one electrode applied with a fixed voltage VG (equal to half theoperating voltage supply Vcc of the memory).

Memory array M-ARY is of the folded bit line system, although this invention is not confined to it, and a pair of rows is shown in FIG. 2. A plurality of memory cells MC have their input/output nodes in a certain orderly arrangement coupled to a pair of complementary data lines DL and DL disposed in parallel.

Precharge circuit PC is formed of an n-channel switching MOS FET, shown by MOS FET Q1 as a typical case, disposed between the complementary data lines DL and DL. Of the complementary data lines DL and DL, one is broughtto the supply voltage Vcc and the other to the ground voltage Vss by a sense amplifier SA as a result of the previous read or write cycle. Prior to the next access cycle, the complementary data lines DL and DL are short-circuited through the MOS FET Q1 in response to a "high" precharge signal PC produced by a timing generating circuit TG. Consequently, a precharge level Vcc/2 for the data lines DL and DL is attained.

The sense amplifier SA consists of p-channel MOS FETs Q2 and Q3 and n-channel MOS FETs Q4 and Q5 in this embodiment, i.e., it consists of a CMOS inverter made up of Q2 and Q4 and another CMOS inverter made up of Q3and Q5, with their inputs and outputs cross-coupled, to form a CMOS latch circuit, and its pair of input/output nodes are coupled to the complementary data lines DL and DL. The latch circuit is supplied with thepower voltage Vcc through parallel-connected p-channel MOS FETs Q6 and Q7 and is also supplied with the ground voltage Vss through parallel-connected n-channel MOS FETs Q8 and Q9. These power switching MOSFET pairs Q6-Q7 and Q8-Q9 are used commonly for other latch circuits provided on other rows in the same memory mat (where, for example, a mat is an arrangement of one or more memory arrays in the memory area of an ICchip). In other words, p-channel MOS FETs and n-channel MOS FETs constituting latch circuits in the same memory mat have their source electrodes connected in common.

The MOS FETs Q8 and Q6 have their gates receiving complementary timing pulses φpal and φpal for activating the sense amplifier SA in the operating cycle, while the MOS FETs Q9 and Q7 have their gates receiving complementary timing pulses φpa2 and φpa2 that are the retarded (delayed) versions of φpal and φpal. By this timing scheme, the sense amplifier SA operates in two stages. In the first stage when the timing pulses φpal and φpal are applied, a small voltage read out of a memory cell to a pair of data lines is amplified without unwanted level fluctuation owing to the current limiting ability of the MOS FETs Q8and Q6 having a relatively small conductance. After the difference of voltages on the complementary data lines has been made greater by the amplification of the sense amplifier SA, it enters the second stage in response to the generation of the timing pulses φpa2 and φpa2, wherein the MOS FETs Q9 and Q7 which have a relatively large conductance are made conductive. The amplifying operation of the sense amplifier SA isspeeded up by the switching of the MOS FETs Q9 and Q7 from the nonconductive to the conductive state. Through the 2-stage amplifying operation of the sense amplifier SA, data can be read out quickly while atthe same time being protected from unwanted voltage level fluctuation on the complementary data lines.

In the case where the voltage transferred by the memory cell MC to the dataline DL is higher (or lower) than the precharge voltage Vcc/2, the sense amplifier SA brings the data line DL to the power voltage Vcc (or ground voltage Vss). As a result of the differential amplification by the sense amplifier SA, the complementary data lines DL and DL will eventually have the power voltage Vcc and ground voltage Vss, respectively, or vice versa.

Row address decoder R-DCR decodes the internal complementary address signalax0-axn-1 provided by a row address buffer R-ADB for implementing the word line selection in response to the word line select timing signal φx which is produced by a timing circuit TG described later.

The row address buffer R-ADB receives the row address signal AX0-AXn supplied on external terminals in response to the timing signal φar produced from the row address strobe signal RAS by the timing generating circuit TG. From the address signal AX0-AXn, the row address buffer R-ADB produces an internal address signal which is in phase with the address signal AX0-AXn and an internal address signal which is out of phase with AX0-AXn (these internal address signals will be termed comprehensively "internal complementary address signal ax0-axn", and this rule will be applied equally to other internal address signals in the following description and illustration).

Column switch C-SW consists of MOS FETs Q10 and Q11 in this embodiment, andit connects the complementary data lines DL and DL with common complementary data lines CD and CD selectively. The MOS FETs Q10 and Q11 have their gates receiving the select signals from a column decoder C-DCR.The column decoder C-DCR produces a data line select signal for selecting one complementary pair of data lines and sends the select signal to the column switch C-SW. The column address decoder C-DCR decodes the internal complementary address signal ay0-ayn-1 provided by a column address bufferC-ADB, which will be described later, and implements a certain data line selecting operation in response to the data line selection timing signal φy.

Column address buffer C-ADB takes the column address signal AY0-AYn on the external terminals in response to the timing signal φac which is produced from the column address strobe signal CAS by the timing generating circuit TG. The column address buffer C-ADB produces the internal complementary address signal ay0-ayn from the address signal AY0-AYn.

The dynamic RAM device of this embodiment includes four memory arrays ARY, although the present invention is not limited to this number. Each memory array has a storage capacity of about 1M bits.

Accordingly, the dynamic RAM of this embodiment has a total storage capacity as large as 4M bits. Four pairs of complementary data lines for the four memory arrays are grouped, and a data line select signal is alloted to these lines. The four pairs of complementary data lines are coupled through the column switch circuit C-SW to four pairs of common complementary data lines CD0, CD1, CD2 and CD3. It should be noted that a non-inverted common data line CD0 and inverted common data line CD0 are expressed comprehensively as a common complementary data line CD0.

The complementary address signals ax0-axn and ay0-ayn have a specific bit, e.g., the highest-order bits axn and ayn, fed to a decoder circuit DEC. The decoder circuit DEC produces from the bit signals axn and ayn the select signals to be given to multiplexers MPX1 and MPX2 in an input circuit and output circuit, respectively, for the signals described later.

The common complementary data lines CD0-CD3 are coupled to the input terminals of respective main amplifiers MA0-MA3, which amplify the signalson the common complementary data lines CD0-CD3 activated by a main amplifier operation timing signal (not shown) produced by the timing generating circuit TG. The main amplifiers MA0-MA3 have their complementary output signals selected by a multiplexer MPX1 which is controlled by the select signal provided by the decoder circuit DEC, and aselected amplifier output signal is delivered to the input terminal of a data output buffer DOB. In the normal operating mode with the test signal TE being at a logic "low" level, the multiplexer MPX1 selects an output signal from among the main amplifiers MA0-MA3 in accordance with the output signal of the decoder circuit DEC. A complementary signal selected by the multiplexer MPX1 is delivered to the input terminal (an input terminal of the data output buffer DOB) of an output circuit OC which constitutes the data output buffer DOB. The output circuit OC is activatedby the timing signal φrw to amplify the input signal, and sends its output to an external terminal Dout for the bit-unit readout operation. The timing signal φrw is produced by the timing control circuit TC during the read mode when the write enable signal WE is "high". During thewrite mode, the output of the output circuit OC, i.e., data output buffer DOB, is placed in a high impedance state by the signal φrw.

The common complementary data lines CD0-CD3 are coupled to the output terminal of a data input circuit DIB through a multiplexer MPX2 which serves as an input selecting circuit. In the normal operating mode, the multiplexer MPX2 is controlled by the select signal produced by the decoder circuit DEC, and delivers the complementary output signal of the data input circuit DIB to a selected one of the common complementary data lines CD0-CD3. The data input circuit DIB is activated by the timing signal φrw, and delivers the input signal on the external terminal Dinto a selected one of the common complementary data lines CD0-CD3 through the multiplexer MPX2 for the bit-unit write operation. The timing signal φrw is generated by the timing generating circuit TG with a certain delay following the main amplifier operation timing signal during the write mode when the write enable signal WE is "low". In the read mode, theoutput of the data input circuit DIB is placed in a high impedance state bythe signal φrw.

The timing generating circuit TG produces the above-mentioned various timing signals necessary for the memory operation by receiving three external control signals RAS (row address strobe signal), CAS (column address strobe signal) and WE (write enable signal).

The dynamic RAM of this embodiment incorporates a test circuit in order to reduce time expended for verifying the capacious memory device. The test circuit on the part of data input is included in the multiplexer MPX2. In the test period or in the test mode when the test signal TE is "high", thetest circuit enables all outputs of the multiplexer MPX2 to introduce the input signal on the external terminal Din onto the common complementary data lines CD0-CD3. Consequently, the same signal is written concurrently on four selected memory cells in each of the plurality (for example, four (4)) of memory arrays M-ARY. Namely, a 4-bit data is thus written into thememory during the test mode.

The test circuit may be a switch circuit (e.g., MOS FET switch) provided inparallel to each unit circuit of the multiplexer MPX2, which is made conductive by a "high" test signal TE. Each unit circuit of the multiplexer MPX2 may be deactivated in the test mode.

The test circuit on the part of data output is included in the multiplexer MPX1 and data output buffer DOB. In the test period or in the test operation when the signal TE is "high", the test circuit in the multiplexer PX1 enables all outputs of the multiplexer MPX1 so that the output signals of the main amplifiers MA0-MA3 are introduced to a judgement or determination circuit JC. The test circuit may be a switch circuit (e.g., MOS FET switch) provided in parallel to each unit circuit of the multiplexer MPX1, which is made conductive by a "high" test signal TE. In the test mode, each unit circuit of the multiplexer MPX1 is deactivated and its output to the output circuit OC is placed in a high impedance state.

The judgement or determination circuit JC is a test circuit included in thedata output buffer DOB which also includes the data output circuit OC. The judgement or determination circuit JC is activated by the test signal TE. The circuit JC receives the output signals of the main amplifiers MA0-MA3 to detect (judge) whether there is consistency or inconsistency of the signals, and issues (e.g., generates) an output signal, depending on the detection result, to the external terminal Dout by way of the output circuit OC. Accordingly, the circuit operates such that it reads 4-bit data during the test mode.

The judgement or determination circuit JC is configured by exclusive-OR (orNOR) circuits, for example. The outputs of the main amplifiers MA0 and MA1 and the outputs of the main amplifiers MA2 and MA3 are compared by first and second exclusive-OR circuits, which have their outputs compared with each other by a third exclusive-OR circuit. The judgement or determinationcircuit JC issues an output signal based on the output of the third exclusive-OR circuit to the output circuit OC. Consequently, the output circuit OC produces a "high" or "low" output signal when all of the 4-bit readout signals from the main amplifiers MA0-MA3 are either at a logic "high" or logic "low" level, respectively. Of the 4-bit readout signals, if any bit is inconsistent with the others, the output terminal Dout is brought to a high impedance state.

In case of a fault or error in each and every one of the selected memory cells being tested, for example, wherein stored data of all 4-bit memory cells are altered thereby changing the logic states thereof, the output circuit OC issues a "high" or "low" output as if no fault or error has occured. On this account, it is desirable for the tester to hold input data according to the expected value and to compare the readout signal with the expected value.

Activation and deactivation of the test circuit is controlled by the test signal TE provided by a latch circuit FF which is set or reset by the operating mode identification output from the timing generating circuit TG. For example, when the test signal TE is "high", each test circuit is activated, and when the test signal is "low", the test circuit is deactivated. Switching of the test mode and normal mode takes place in this way.

Activation and deactivation of the test circuit will further be described with reference to the timing chart shown in FIG. 1.

At the transitional logic level corresponding to the trailing or falling edge of the row address strobe signal RAS, the column address strobe signal CAS and write enable signal WE are both at a "low" logic level. In response to this transition, the timing generating circuit TG sends a "high" signal to the latch circuit FF. Then, the latch circuit FF is set to produce a "high" test signal TE. Namely, this memory cycle implements only the setting of the test mode.

For example, when the dynamic RAM incorporates an automatic refreshing circuit of the type of CAS-before-RAS refreshing, the refreshing operationtakes place concurrently with the setting of the test mode on the basis of the relation between the address strobe signals RAS and CAS. Such a concurrent occurrence of test mode setting and refreshing may be avoided by disabling the refresh mode using a "low" write enable signal WE.

For the write/read operation in the actual testing, the RAS and CAS signalsare once brought to "high" to reset the dynamic RAM. This is followed by the read/write operation in the normal mode. With the row address strobe signal RAS being placed "low", a row address signal AX0-AXn is taken in, and thereafter with the column address signal CAS being placed "low", a column address signal AY0-AYn is taken in. Following the signal φar, signals φx, φpa (φpa1, φpa1, φpa2 and φpa2) and a main amplifier operation signal (not shown) are produced at a certain timing relationship. Signal φy is produced following signal φac. As a result, four memory cells corresponding to address signals ax0-axn-1 and ay0-ayn-1 are connected to the common data lines CD0-CD3.

At this time, for writing test data, the write enable signal WE is made "low" at the timing shown in the figure. Consequently, the generated signals φrw and φrw make the data input buffer DIB active and the output circuit OC inactive. Since the test signal TE is "high", the entiremultiplier MPX2 is enabled and a data input signal supplied at the externalterminal Din is introduced by way of the data input buffer DIB and multiplexer MPX2 to the common data lines CD0-CD3. As a result, a piece (or bit) of data is written in the four memory cells, i.e., 4-bit writing takes place. The signals of a pair of complementary signals provided at the inputs of a main amplifier have a voltage difference of about 200 mV, for example, and that outputs of the data input buffer DIB are as large asabout 5 volts. On this account, data on the external terminal Din is written into memory cells irrespective of the operation of the main amplifier.

Subsequently, test data which has been written in the memory cells is read out. In the same way as described above, four memory cells corresponding to the address signal ax0-axn-1 and ay0-ayn-1 are connected to the common data lines CD0-CD3 in the normal mode. At this time, for reading out test data, the write enable signal WE is made "high" as shown by the dashed line in FIG. 1. Consequently, the generated signals φrw and φrw make the data input buffer DIB inactive and the output circuit OC active. Since the test signal TE is "high", the multiplexer MPX1 delivers the output signals of the main amplifiers MA0-MA3 to the judgement or determination circuit JC, and at the same time a selected output which is coupled to the output circuit OC is brought to a high impedance state. In response to the "high" test signal TE, the judgement circuit tests whetheror not the 4-bit signals are consistent, i.e. in agreement. The output circuit OC responds to the result determined by the judgement circuit so as to bring the external terminal Dout to a logic "high", a logic "low" orto a high impedance state, accordingly. Thus, 4-bit data reading takes place. It is thus possible to determine the presence of a defective bit inthe selected four memory cells.

The memory test with the test signal TE being "high" can be cycled without lowering the test signal TE each time, although this invention is not confined to this scheme. Reading may take place repeatedly following the 4-bit writing of test data. After writing test data in all bits or all bits of one memory array, the bit data may be read out.

On completion of the test, the test mode is terminated. In order for this operation to take place, the column address strobe CAS and write enable signal WE are made "low" and "high", respectively, at the falling edge of the row address strobe signal RAS. In response to this transition, the timing generating circuit TG sends a "low" signal to the latch circuit FF.Then, the latch circuit FF is set, and the test signal TE is made "low". Inthis memory cycle RESET, only cancellation of the test mode takes place.

For example, when the dynamic RAM incorporates an automatic refreshing circuit of the type of CAS-before-RAS refreshing, the refreshing operationtakes place concurrently with the test mode cancellation on the basis of the relationship between the address strobe signals RAS and CAS.

Accordingly, the test signal TE can be made "low", and the following operation can take place in the normal mode. On this account, the RAS and CAS signals are made "high", and the dynamic RAM is reset.

The effectiveness achieved by the foregoing embodiment is as follows.

(1) By the combinations of the row address and column address strobe signals with the write enable signal, which combinations are unused in thenormal mode, the test mode can be initiated and cancelled without increasing the number of external control signals.

(2) Accordingly, a dynamic RAM as capacious as 4M bits can be accommodated in an 18-pin package. This enables matching with a 1M-bit dynamic RAM, while adding the test function.

Although the present invention has been described with respect to a specific embodiment, the invention is not to be construed as being confined to this embodiment only because various changes are of course possible without departing from the spirit and scope thereof. For example,the address signal may be included in the combinations of the signals RAS, CAS and WE for the setting and cancellation of the test mode.

As shown by the dashed line in FIG. 2, the latch circuit FF receives a signal ai through a specific external address input terminal Ai. The timing generating circuit TG produces a one-shot pulse in response to a "low" column address strobe signal CAS and write enable signal WE at the transitional falling edge of the row address strobe signal RAS. The latch circuit FF responds to this one-shot pulse to accept a signal at the specific address terminal. For example, when address terminal Ai provides a "high" signal, as shown in FIG. 3, the test mode is set, i.e., the test signal TE is made "high". The signal ai is supplied from the row address buffer R-ADB, although the present invention is not confined to this scheme.

On completion of the memory cycle SET for setting the test mode, the test cycle TEST is repeated. After the test, the memory cycle RESET for cancelling the test mode is carried out as follows. The timing generating circuit TG produces a one-shot pulse depending on the combination of the signals RAS, CAS and WE, as used in the memory cycle SET, as shown in FIG.3. The latch circuit FF responds to this one-shot pulse to accept a "low" signal on the address terminal Ai. Consequently, the test signal is made "low", and the test mode is cancelled.

Besides the initiation and cancellation of the test mode, it is also possible to provide the data output buffer DOB with a function of outputting the inconsistency output signal in terms of a selected mode defined by a high impedance state and an intermediate signal level condition (mid-point voltage 1/2 Vcc between the power voltage Vcc and ground voltage Vss), and operate the data output buffer DOB so that such mode is selected. By the addition of the above function, the inconsistencyoutput signal can be switched depending on the tester used. For example, with the dynamic RAM mounted on a memory board, the output terminal Dout is connected in a wired-OR configuration by the data bus on the board. Since the data bus still retains the signal of the previous operating cycle, the inconsistency output signal in the high-impedance output mode makes identification difficult. In such a case, the signal can be switchedto the intermediate-level output mode for testing a dynamic RAM on the memory board.

Selection of the output mode can be accomplished using the address signal. In the memory cycle SET in FIG. 3, the signal (address signal) received onthe external terminal Ai-1, as shown by the dashed line, is latched in a latch circuit (not shown). The signal on the external terminal Ai-1 is made effective only when the signal on the external terminal Ai is "high" during the memory cycles SET and RESET. When the latch circuit provides a "high" or "low" output, the output circuit OC brings the inconsistency signal to the high-impedance or intermediate level, respectively.

In case the output circuit OC has its final output stage made up of first and second MOS FETs connected between the power voltage Vcc and external terminal Dout and between the ground voltage Vss and external terminal Dout, the selection of output mode of the data output buffer DOB takes place as follows.

In the normal output mode, the first circuit in the output circuit OC supplies complementary signals to the gates of the first and second MOS FETs. The first circuit is activated or deactivated in response to a "low"or "high" test signal TE, respectively. When outputting the inconsistency signal ("high" or "low") in the test mode, the second circuit in the output circuit OC supplies complementary signals to the gates of the firstand second MOS FETs. The output circuit OC further includes third and fourth circuits for the inconsistency output in the test mode. Upon receiving the inconsistency signal, the third circuit supplies a "low" signal to the gates of the first and second MOS FETs. Then, the two MOS FETs are cut off, and the external terminal Dout is rendered in a high impedance state condition. The fourth circuit, upon receiving the inconsistency signal, supplies a "high" signal to the gates of the first and second MOS FETS. Then the two output MOS FETs become conductive, causing the external terminal Dout to have a voltage depending on the conductance (gm) of the two MOS FETs, e.g., 1/2 Vcc.

In practice, each pair of the second and third circuits and the second and fourth circuits is constructed in a unitary circuit. When the test signal TE is "high", one of these signals is made active by the signal on the external terminal Ai-l.

The address terminal Ai is the highest-order bit of the address signal, e.g., terminal A10 for a 1M-bit RAM. In this embodiment, the terminal Ai is terminal An for the internal signal axn. This terminal designation facilitates the alteration of the chip function. For example, when a 1M-bit RAM chip is configured in 256K words by 4 bits, the terminal A10 isrendered unnecessary. By application of this invention to this case, no change is required for the terminal A10, and it can be used exclusively for mode setting.

The output mode may be selected depending on the signal on terminal Ai-l asfollows. On reception of a "high" signal on the terminal Ai-l, any of "high", "low" and high impedance (or intermediate level) signal is supplied to the external terminal Dout. With a "low" signal being given, a "high" signal and a "low" signal are supplied as the consistency signal and inconsistency signal, respectively, to the external terminal Dout.

By combining the address signal with the row address strobe signal, column address strobe signal and write enable signal, test mode initiation and cancellation can be simplified, and a test function with multiple modes can be added.

Instead of the address terminals Ai and Ai-l, the input terminal Din or output terminal Dout may be used. Cancellation of the test mode may take place in response to the sole transition of the RAS signal to "low" in onememory cycle.

The latch circuit FF may be constructed by a binary counter circuit using master-slave flip-flops. The counter circuit operates in response to a one-shot pulse produced by the timing generating circuit TG by placing thecolumn address strobe signal CAS and write enable signal WE at "low" level at the falling edge of the row address strobe signal RAS. Depending on theoutput of the counter circuit, either the test mode or normal mode is selected. Preferably, the counter circuit is designed so that the RAM chipis set to the test mode or normal mode invariably when power is turned on.

The dynamic RAM to which this invention is applied may be one having a nibble mode function in which a signal with multiple bits read out in parallel from the memory array is outputted in a serial fashion in response to a signal which alternates in synchronism with the column address strobe signal. In this case, the address signal supplied to the decoder circuit DEC in FIG. 2 is controlled by a shift register or addresscounter circuit. The memory array M-ARY may be constructed with a reduced number of memory cells coupled with the word lines and/or data lines, and in the form of multiple memory mats so as to speedup and enhance the levelmargin of the signals read out of the memory cells.

The number of memory cells selected in other words the number of common complemetary data lines, may take any value larger than one, such as 8 bits or 16 bits in addition to the foregoing case of 4 bits. Moreover, if some pins are left unused when the present invention is applied to a dynamic RAM having a storage capacity of 1M bits or 256K bits, they may beused for other operating modes. The present invention can be used extensively for dynamic RAM devices incorporating a test circuit. 

What is claimed is:
 1. An address multiplexed dynamic random access memory (RAM) having both a normal operation mode and a test mode comprising:a first external terminal for receiving a row address strobe (RAS) signal; a second external terminal for receiving a column address strobe (CAS) signal; a third external terminal for receiving a write enable (WE) signal; external address terminals for receiving row address signals and column address signals; a row address buffer coupled to said external address terminals; and a column address buffer coupled to said external address terminals, wherein said test mode is initiated in response to said CAS signal being at a logic "low" level and said WE signal being at a logic "low" level when said RAS signal is at a transitional logic level corresponding to a falling edge, wherein, in said test mode, said row address buffer responds to said row address signals in response to said RAS signal being at a transitional logic level corresponding to a falling edge and said column address buffer responds to said column address signals in response to said CAS signal being at a transitional logic level corresponding to a falling edge, and wherein said dynamic RAM is returned to said normal operation mode in response to said CAS signal being at a logic "low" level and said WE signal being at a logic "high" level when said RAS signal is at a transitional logic level corresponding to a falling edge.
 2. An address multiplexed dynamic RAM according to claim 1, further comprising:testing means, which is coupled to receive a plurality of signals read out from selected memory cells, included in said dynamic RAM, on the basis of said row address signals and said column address signals, and which is activated in said test mode, for judging whether there is consistency or inconsistency of said plurality of signals which are read out.
 3. An address multiplexed dynamic RAM according to claim 2, wherein said testing means produces an output signal which is of a first logic level when there is consistency of said plurality of signals and which is of a second logic level when there is inconsistency of said plurality of signals.
 4. An address multiplexed dynamic RAM according to claim 3, further comprising:a plurality of common data lines each of which corresponds to a respective one of plural memory cell arrays included in said dynamic RAM; and a plurality of amplifiers each of which is provided with an input coupled to a respective one of said plurality of common data lines, wherein said plurality of amplifiers provide output signals, indicative of said plurality of signals read out from the selected memory cells, to said testing means during said test mode.
 5. An address multiplexed dynamic RAM according to claim 4, further comprising:latch means for supplying an output signal to said testing means indicative of whether said dynamic RAM, while operational, is in said normal operation mode or in said test mode.
 6. An address multiplexed dynamic RAM according to claim 5, further comprising:means for selecting one or more of said output signals of said plurality of amplifiers; and decode means for controlling said means for selecting on the basis of at least one of said column address signals.
 7. An address multiplexed dynamic RAM according to claim 6, further comprising:a fourth external terminal coupled to said means for selecting and said testing means and providing output data indicative of an output signal of a selected one of said plurality of amplifiers, via said means for selecting, in said normal operation mode and output data corresponding to the output signal of said testing means in said test mode.
 8. An address multiplexed dynamic RAM according to claim 7, wherein said first logic level is a logic "high" level and said second logic level is a logic "low" level.
 9. An address multiplexed dynamic RAM according to claim 8, further comprising:an output circuit having an output terminal coupled to said fourth external terminal and an input terminal coupled to both an output terminal of said testing means and an output terminal of said means for selecting.
 10. An address multiplexed dynamic RAM according to claim 8, wherein said means for selecting includes a multiplexer.
 11. An address multiplexed dynamic random access memory (RAM) having both a normal operation mode and a test mode comprising:a first external terminal for receiving a row address strobe (RAS) signal; a second external terminal for receiving a column address strobe (CAS) signal; a third external terminal for receiving a write enable (WE) signal; a fourth external terminal for receiving input data; external address terminals for receiving row address signals and column address signals; a row address buffer coupled to said external address terminals; and a column address buffer coupled to said external address terminals, wherein said test mode is initiated in response to said CAS signal being at a logic "low" level and said WE signal being at a logic "low" level when said RAS signal is at a transitional logic level corresponding to a falling edge; wherein, in said test mode, said row address buffer responds to said row address signals in response to said RAS signal being at a transitional logic level corresponding to a falling edge and said column address buffer responds to said column address signals in response to said CAS signal being at a transitional logic level corresponding to a falling edge; wherein, in said test mode, the same input data is written in a plurality of memory cells of said dynamic RAM which are selected on the basis of said row address signals and said column address signals when said WE signal is at a logic "low" level, wherein, in said test mode, a plurality of signals are read out from said plurality of memory cells on the basis of said row address signals and said column address signals when said WE signal is at a logic "high" level, and wherein said dynamic RAM is returned to said normal operation mode in response to said CAS signal being at a logic "low" level and said WE signal being at a logic "high" level when said RAS signal is at a transitional logic level corresponding to a falling edge.
 12. An address multiplexed dynamic RAM according to claim 11, further comprising:testing means, which is coupled to receive said plurality of signals which are read out from said plurality of memory cells and which is activated in said test mode, for judging whether there is consistency or inconsistency of said plurality of signals which are read out.
 13. An address multiplexed dynamic RAM according to claim 12, wherein said testing means produces an output signal which is of a first logic level when there is consistency of said plurality of signals and which is of a second logic level when there is inconsistency of said plurality of signals.
 14. An address multiplexed dynamic RAM according to claim 13, further comprising:a plurality of common data lines each of which corresponds to a respective one of plural memory cell arrays included in said dynamic RAM; and a plurality of amplifiers each of which is provided with an input coupled to a respective one of said plurality of common data lines, wherein said plurality of amplifiers provide output signals, indicative of said plurality of signals read out from said plurality of memory cells, to said testing means during said test mode.
 15. An address multiplexed dynamic RAM according to claim 14, further comprising:latch means for supplying an output signal to said testing means indicative of whether said dynamic RAM, while operational, is in said normal operation mode or in said test mode.
 16. An address multiplexed dynamic RAM according to claim 15, further comprising:first means for selecting one or more of said output signals of said plurality of amplifiers; and decode means for controlling said first means for selecting on the basis of at least one of said column address signals.
 17. An address multiplexed dynamic RAM according to claim 16, further comprising:a fifth external terminal coupled to said first means for selecting and said testing means and providing output data indicative of an output signal of a selected one of said plurality of amplifiers, via said first means for selecting, in said normal operation mode and output data corresponding to the output signal of said testing means in said test mode.
 18. An address multiplexed dynamic RAM according to claim 17, wherein said first logic level is a logic "high" level and said second logic level is a logic "low" level.
 19. An address multiplexed dynamic RAM according to claim 18, further comprising:an output circuit having an output terminal coupled to said fifth external terminal and an input terminal coupled to both an output terminal of said testing means and an output terminal of said first means for selecting.
 20. An address multiplexed dynamic RAM according to claim 19, wherein said first means for selecting includes a multiplexer.
 21. An address multiplexed dynamic RAM according to claim 19, further comprising:an input circuit having an input terminal coupled to said fourth external terminal and output terminal coupled to one or more of said plurality of common data lines.
 22. An address multiplexed dynamic RAM according to claim 21, further comprising:second means, coupled between said input circuit and said plurality of common data lines, for transmitting an output signal of said input circuit to one of said plurality of common data lines in said normal operation mode and to all of said plurality of common data lines in said test mode.
 23. An address multiplexed dynamic RAM according to claim 22, wherein said second means for transmitting an output signal of said input circuit is controlled by an output signal of said decode means for selecting one of said plurality of common data lines in said normal operation mode.
 24. An address multiplexed dynamic RAM according to claim 23, wherein said first means for selecting and said second means for transmitting an output signal of said input circuit include a first multiplexer and a second multiplexer, respectively.
 25. An address multiplexed dynamic RAM according to claim 22, wherein in said test mode said second means for transmitting an output signal of said input circuit is controlled by said output signal of said latch means in which all of said plurality of common data lines are selected.
 26. An address multiplexed dynamic RAM according to claim 25, wherein said first means for selecting and said second means for transmitting an output signal of said input circuit include a first multiplexer and a second multiplexer, respectively.
 27. An address multiplexed dynamic random access memory (RAM) having both a normal operation mode and a test mode comprising:a first external terminal for receiving a row address strobe (RAS) signal having first and second voltage levels indicative of binary logic levels "1" and "0", respectively; a second external terminal for receiving a column address strobe (CAS) signal having corresponding first and second voltage levels indicative of binary logic levels "1" and "0", respectively; a third external terminal for receiving a write enable (WE) signal having corresponding first and second voltage levels indicative of binary logic levels "1" and "0", respectively; external address terminals for receiving row address signals and column address signals; a row address buffer coupled to said external address terminals and being activated by a row address buffer timing signal produced in response to said RAS signal; and a column address buffer coupled to said external address terminals and being activated by a column address buffer timing signal produced in response to said CAS signal, wherein said test mode is initiated in response to said CAS signal being at a logic "0" level and said WE signal being at a logic "0" level when said RAS signal is at a transitional logic level from that of said logic "1" level to said logic "0" level, wherein, in said test mode, said row address buffer responds to said row address signals in response to said RAS signal being at a transitional logic level from that of said logic "1" level to said logic "0" level and said column address buffer responds to said column address signals in response to said CAS signal being at a transitional logic level from that of said logic "1" level to said logic "0" level, and wherein said dynamic RAM is returned to said normal operation mode in response to said CAS signal being at a logic "0" level and said WE signal being at a logic "1" level when said RAS signal is at a transitional logic level from that of said logic "1" level to said logic "0" level.
 28. An address multiplexed dynamic RAM according to claim 27, further comprising:testing means, which is coupled to receive a plurality of signals which are read out from selected memory cells, included in said dynamic RAM, on the basis of said row address signals and said column address signals, and which is activated in said test mode, for judging whether there is consistency or inconsistency of said plurality of signals which are read out.
 29. An address multiplexed dynamic random access memory (RAM) having both a normal operational mode and a test mode comprising:a first external terminal for receiving a row address strobe (RAS) signal having first and second voltage levels indicative of binary logic levels "1" and "0", respectively; a second external terminal for receiving a column address strobe (CAS) signal having corresponding first and second voltage levels indicative of binary levels "1" and "0", respectively; a third external terminal for receiving a write enable (WE) signal having corresponding first and second voltage levels indicative of binary logic levels "1" and "0", respectively; a fourth external terminal for receiving input data; external address terminals for receiving row address signals and column address signals; a row address buffer coupled to said external address terminals and being activated by a row address buffer timing signal produced in response to said RAS signal; and a column address buffer coupled to said external address terminals and being activated by a column address buffer timing signal produced in response to said CAS signal, wherein said test mode is initiated in response to said CAS signal being at a logic "0" level and said WE signal being at a logic "0" level when said RAS is at a transitional logic level from that of said logic level "1" to said logic level "0", wherein, in said test mode, said row address buffer responds to said row address signals in response to said RAS signal being at a transitional logic level from that of said logic level "1" to said logic level "0" and said column address buffer responds to said column address signals in response to said CAS signal being at a transitional logic level from that of said logic level "1" to said logic level "0", wherein, in said test mode, the same input data is written in a plurality of memory cells of said dynamic RAM which are selected on the basis of said row address signals and said column address signals when said WE signal is at a logic "0" level; wherein, in said test mode, a plurality of signals are read out from said plurality of memory cells on the basis of said row address signals and said column address signals when said WE signal is at a logic "1" level; and wherein said dynamic RAM is returned to said normal operation mode in response to said CAS signal being at a logic "0" level and said WE signal being at a logic "1" level when said RAS signal is at a transitional logic level from that of said logic level "1" to said logic level "0".
 30. An address multiplexed dynamic RAM according to claim 29, further comprising:testing means, which is coupled to receive said plurality of signals which are read out from said plurality of memory cells and which is activated in said test mode, for judging whether there is consistency or inconsistency of said plurality of signals which are read out.
 31. A method of testing memory cells in an address multiplexed dynamic random access memory (RAM) including a first external terminal for receiving a row address strobe (RAS) signal having first and second voltage levels indicative of binary logic levels "1" and "0", respectively; a second external terminal for receiving a column address strobe (CAS) signal having corresponding first and second voltage levels indicative of binary logic levels "1" and "0", respectively; a third external terminal for receiving a write enable (WE) signal having corresponding first and second voltage levels indicative of binary logic levels "1" and "0", respectively; external address terminals for receiving row address signals and column address signals; and row and column address buffers, coupled to said external address terminals, for providing internal row and column address signals used for accessing memory cells, said method comprising:initiating a test mode in said dynamic RAM in response to said CAS signal being at a logic "0" level and said WE signal being at a logic "0" level when said RAS signal is at a transitional logic level from that of said logic "1" level to said logic "0" level; effecting a testing operation of said dynamic RAM by activating said row address buffer to respond to incoming row address signals in response to said RAS signal being at a transitional logic level from that of said logic "1" level to said logic "0" level and activating said column address buffer to respond to incoming column address signals in response to said CAS signal being at a transitional logic level from that of said logic "1" level to said logic "0" level; and returning said dynamic RAM to a normal operation mode from said test mode in response to said CAS signal being at a logic "0" level and said WE signal being at a logic "1" level when said RAS signal is at a transitional logic level from that of said logic "1" level to said logic "0" level.
 32. A method according to claim 31, wherein in said test mode the same input data is written in parallel into a plurality of memory cells which are selected on the basis of said internal row and column address signals when said WE signal is at a logic "0" level, and wherein in said test mode a plurality of signals are read out from said plurality of memory cells on the basis of said internal row and column address signals when said WE signal is at a logic "0" level.
 33. A method according to claim 32, further comprising a step of:generating an output signal indicating whether there is consistency or inconsistency of said plurality of signals which are read out from said plurality of memory cells during said testing operation. 